1. [Field of the Invention]
The present invention relates to an optical member made of synthetic silica glass of high-purity and transparency, and a method for manufacture of a blank thereof or the optical member. The invention relates more especially to optical members assembled in various apparatuses employing such high-power laser beams as an excimer laser, or optical members assembled in apparatuses used in a radiation environment where they are exposed to gamma rays, X-rays or cosmic rays.
2. [Description of the Prior Art]
In recent years it is noted that high-power lasers such as the excimer laser, etc. are applicable to a lithographic technique for producing large scale integrated circuits (LSIs), a technique applying photochemistry, a processing technique for cutting and grinding, or laser nuclear fusion.
Attempts have been made to apply silica glass optical members such as lenses, prisms, mirrors, or optical fibers not only for communication but also for light or energy transmissions, or other optics, to transmit, to refract, to reflect, to absorb or to interfere with the high-power laser beams.
Attempts have also been tried to apply the silica glass optical members such as lenses, prisms, mirrors, fibers and so forth assembled in an optical apparatus for spacecraft, or an optical apparatus for nuclear reactor periphery operation in a radiation environment where they are exposed to gamma rays, X-rays or cosmic rays.
In the event, however, when the various optics made of silica glass are irradiated with visible light having wave lengths ranging from about 700 to 600 nm, ultraviolet light having wave lengths ranging from about 360 to 160 nm, or ionized radiation beams, the optics are apt to suffer structural damage.
An absorption band of wave length about 630 nm indicates the presence of the defect called a non-bridging oxygen hole center (NBOHC). An absorption band of wave length about 215 nm is the so called E' center. And there is another absorption band of wave length about 260 nm. Those absorption bands are generated upon irradiation with high-power laser beams over a long period, resulting in reducing the optical transmission for the visible light having wave lengths ranging about from 750 to 500 nm, and for the ultraviolet light having wave lengths ranging from about 360 to 160 nm to deteriorate the optical properties of the material.
It is, therefore, very difficult to improve the durability of silica glass against the high-power laser irradiation of beams which have the wave lengths described above, because the glass suffers the structural damage.
Further, it is confirmed that among the various types of lasers, the KrF excimer laser and the ArF excimer laser are the most powerful in subjecting the silica glass to optical damage.
The present inventors have first tried to produce an optical member with a fair measure of success from a standard synthetic silica glass of high-purity and high-homogeneity.
The inventors' study proceeded to develop the technique filed by the inventors as Japanese Pat. Appln. 1-145226, which is referred to as a prior application technique hereinafter, for doping with hydrogen in the synthetic silica glass described above as a basic material to suppress drastically the optical damage on the irradiation of ultraviolet light having a wave length less than about 250 nm.
In the doping method of the prior application technique, however, hydrogen is more doped in the surface region of the glass in contact with the dopant gas. It is rather difficult to dope homogeneously in the entire solid glass even in an atmosphere of elevated temperature. The difficulty tends to be amplified with the increase of the thickness of the glass to be doped. In this doping method, therefore, the laser damage prevention effect is limited to a certain maximum thickness.
Because the glass members for laser use should be high-purity and high-homogeneity synthetic glass, other starting glasses can not be used as the starting materials. Since glass in the synthetic, process is subjected within a short time to an elevated temperature of an oxyhydrogen flame, the reaction process seemingly is not sufficient to reach an equilibrium state to produce a stable structure. Therefore, the synthetic glass includes a glass structure of three member rings or four member rings which are not detected in quartz. It is believed that the unstable structures may contribute to the decrease of durability against laser irradiation. It is confirmed that the silica glass having the higher scattering peaks of wave numbers of 495 and 606 cm.sup.-1 in laser Raman scattering spectrometry is inferior in laser durability.
To obviate the issue by the inventors as described above, the structure has been reformed with hydrogen doping. The procedure was merely a relief remedy not solving the issue substantially, but instead including the limitation on feasible thickness for doping as stated previously.
Once the synthetic silica glass has suffered the light or irradiation damage, a chemical bond between silicon and oxygen in the network structure generally is broken, which rejoins with other structures thus increasing the density and the local absolute refractive index.
Even when the refractive index is increased due to the breakage and recombination of the bonds between silicon and oxygen, with the presence of hydrogen molecules near the split bond, an OH group is formed by reaction with hydrogen molecules provided in the silica glass, whereby most of the split bonds are remedied to suppress the increase of refractive index. There still remains another issue to be solved when the silica glass is exposed to the high-power laser beam for a long period.
That is, though the initial deterioration can be improved with the presence of hydrogen, a further irradiation with the high-power laser beam causes exhaustion of the hydrogen within a short time, even if the hydrogen is included a concentration of more than 5 .times.10.sup.16 molecules/cm.sup.3. No remedial effect can be expected with a hydrogen concentration of less than 1.times.10.sup.16 molecules/cm.sup.3.
In the prior technique as described above, the technique for doping the silica glass with hydrogen is disclosed for the treatment in atmospheric or high pressure gas at the elevated temperature from 200.degree. to 1200.degree. C. A similar technique for doping with hydrogen is also disclosed in Japanese Laid Open Pat. Appln. No. 1-201664, showing a technique possible for doping at ambient atmospheric pressure at an elevated temperature from 800.degree. to 1,000.degree. C.
These techniques handling the hydrogen at an elevated temperature involve an explosion hazard for which safety cares must be taken.